Syntheses and Characterization of Tantalum Alkyl Imides and Amide Imides. DFT Studies of Unusual α-SiMe3 Abstraction by an Amide Ligand

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• 作者：Seth C. Hunter ; Shu-Jian Chen ; Carlos A. Steren ; Michael G. Richmond ; Zi-Ling Xue
• 刊名：Organometallics
• 出版年：2015
• 出版时间：December 28, 2015
• 年：2015
• 卷：34
• 期：24
• 页码：5687-5696
• 全文大小：591K
• ISSN：1520-6041

文摘

Reaction of TaCl₂(═NSiMe₃)[N(SiMe₃)₂] (1) with alkylating reagents form the alkyl amide imide complexes TaR₂(═NSiMe₃)[N(SiMe₃)₂] (R = Me (2), CH₂Ph (3)) and mixed amide imide compounds Ta(NR′₂)₂(═NSiMe₃)[N(SiMe₃)₂] (R′ = Me (4), Et (5)). The reaction of 2 and 0.5 equiv of O₂ leads to preferential oxygen insertion into one Ta–Me bond, yielding the alkoxy-bridged alkyl dimer Ta₂(μ-OMe)₂Me₂(═NSiMe₃)₂[N(SiMe₃)₂]₂ (6) as cis and trans isomers. Crystallization of the cis-6 and trans-6 mixture gave only crystals of trans-6. When the crystals of trans-6 were dissolved in benzene-d₆, conversion of trans-6 to cis-6 occurred until the trans-6 ? cis-6 equilibrium was reached with K_eq = 0.79(0.02) at 25.0(0.1) °C. Kinetic studies of the exchange gave the rate constants k = 0.018(0.002) min^–1 for the trans-6 → cis-6 conversion and k′ = 0.022(0.002) min^–1 for the reverse cis-6 → trans-6 conversion at 25.0(0.1) °C. Complex 6 reacts with additional O₂, forming the dialkoxy dimer Ta₂(μ-OMe)₂(OMe)₂(═NSiMe₃)₂[N(SiMe₃)₂]₂ (7) as cis and trans isomers. Solid-state structures of 3 and trans-6 have been determined by X-ray diffraction analyses. The mixed amide imide compounds Ta(NR′₂)₂(═NSiMe₃)[N(SiMe₃)₂] (R′ = Me (4), Et (5)) have also been prepared by salt metathesis reactions employing TaCl₃[N(SiMe₃)₂]₂ (8). The pathway from 8 to 4 and 5 eliminates Me₃Si–NR′₂ (R′ = Me, Et), converting the amide N(SiMe₃)₂ ligand to the imide ═NSiMe₃ ligand. Such intramolecular imidation is rare. The mechanism of this process has been computationally probed, and α-elimination involving the mixed amide species TaCl₂(NMe₂)[N(SiMe₃)₂]₂ (9) is discussed. Diffusion-ordered spectroscopy (DOSY) studies of 1–6 and 8 show that only the alkoxy-bridged cis-6 and trans-6 are dimers in benzene-d₆ solution at 25 °C.